1. Field of the Invention
The present invention pertains generally to transforming a Gaussian laser beam to a far field diffraction pattern, and more particularly to employing an off-axis blazed diffractive phase grating to shape a Gaussian beam to a non-Gaussian intensity shape at some small diffracted angle for a single diffraction order and transforming the intensity shape to its far field pattern in one or two dimensions.
2. Description of the Background Art
Since the time that the laser was invented and employed in labs around the world, scientists have been proposing and experimenting with various beam shaping schemes. For example, the fundamental Gaussian TEM00 profile has great flexibility and utility in many applications. However, without intensity profile shaping as a degree of freedom for design engineers, many applications cannot be realized with sufficient quality. Furthermore, the high cost of early beam shaping techniques limited their practical use. With the recent development of highly efficient diffractive optics, however, beam-shaping techniques are being refined. Diffractive efficiencies greater than 90% and minimal degradation of solutions due to manufacturing tolerances allow beam shaping to be performed at a reasonable cost.
A substantial amount of work has been performed over the past twenty years in the area of beam shaping using diffractive optics. Most of this work has centered around producing shaped beams and studying their free space propagation. A limited amount of work has been performed related to using beam shapers with conventional focusing schemes. The properties of focused beams are of paramount interest to a large portion of existing laser applications engineers.
One currently accepted method for obtaining a focused flattop (uniform) intensity is to shape a Gaussian beam with a diffractive optic and then use a refractive lens to produce the focus. This method has been shown to produce flattop intensity at some intermediate plane, rather than the focal plane of the lens. At the focus of the lens, the Fourier transform of the input intensity is produced. Depending on the design parameters, another intensity pattern (non-uniform) is obtained at the focus. This method has been shown by Dickey et al. (Dickey, F. M. et al., “Gaussian laser beam profile shaping,” Opt. Eng. 35(11), pp. 3285–3295 (1996)) to produce a close to theoretical flattop using a CO2 laser. Ledger et al. (Ledger, J. R. et al., “Laser beam relaying with phase conjugate diffractive optical elements,” App Opt 36(20), pp. 4749–4755 (1997)) as well as Borek et al. (Borek, G. et al., “High performance diffractive optics for beam shaping,” SPIE Proceedings 3633, San Jose, Calif., Jan. 1999) have also independently shown techniques for producing propagating flattop intensity profiles with laser beams using diffractive optics. These flattop propagation techniques used with a focusing lens construct an Airy pattern at the focal plane (Brown, D. R., “Laser Beam Shaping-Theory and Techniques,” New York, Marcel Dekker, Chapter 6, p. 252 (2000)). While this Airy pattern at the focus may prove useful for some applications, in general the pattern is not desirable. If laser users desire to perform work with such a flattop profile, however, they are limited to a small depth of field with a spot diameter some factor larger than the lens can produce.
It will be appreciated that all simple lenses perform a Fourier transform on the input intensity function and that this transform is created at the focal plane of the lens. If a technique can be designed to propagate a far field pattern, and this far field pattern is focused, the Fourier transform will create a uniform intensity profile at the focal plane of the lens.